Transcript Energy
Energy physics Work • 1) has to have the application of a force • 2) movement of something by the force • Work= force X distance – W=Fd • Measured in Nm which is a joule – You can have kilojoule or megajoule just like all other metric system numbers. Power • How fast we do work. • Power= work done/ time • The unit for power is the watt. – Again you can have kilowatt, megawatt…. – 1 horsepower is equal to 0.75kW • Engine rated at 134 hp is a 100-kW engine • Question: If a forklift is replaced with a new forklift that has twice the power, how much greater a load can it lift in the same amount of time? If it lifts the same load, how much faster can it operate? answer • The forklift that delivers twice the power will lift twice the load in the same time, or the same load in half the time. Either way, the owner of the new forklift is happy. Mechanical energy • Energy- the ability to do work – Energy is measured in joules. • Mechanical energy is the total energy in a system. – Kinetic + Potential = mechanical Potential Energy • Stored energy. • Has the potential to do work. • There is potential energy in fossil fuels, batteries, and the food we eat. • Gravitational Potential energy- potential energy due to elevated position. • Gpe= weight X height – PE=mgh Questions • 1. how much work is done on a 100 N boulder that you carry horizontally across a 10 m room? How much PE does the boulder gain? • 2. a. How much work is done on a 100 N boulder when you lift it 1 m? • b. what power is expended if you lift the boulder a distance of 1m in a time of 1 s? • c. what is the gravitational potential energy of the boulder in the lifted position? Answers • 1. You do no work on the boulder moved horizontally, because you apply no force in its direction of motion. It has no more PE across the room than it had initially. • 2. a. you do 100 j of work when you lift it 1m, since Fd=100 X 1= 100 J • b. Power= 100/1= 100 W • c. it depends. Rlative to its starting position, the boulder’s PE is 100 J. Relative to some other reference level, its PE would be some other value. Kinetic Energy • Energy of motion • Kinetic energy= ½ mass X speed2 – KE= ½ mv2 – Fd= ½ mv2 – Work= KE • Work-energy theorem= when work is done energy changes. – Work=∆E Conservation of Energy • Energy cannot be created or destroyed. It can be transformed form one form into another, but the total amount of energy never changes. • Efficiency = actual mechanical advantage/ theoretical mechanical advantage. – Efficiency= work output/ work input Questions • A child on a sled (total weight 500 N) is pulled up a 10 m slope that elevates her a vertical distance of 1m. – A. what is the theoretical mechanical advantage of the slope? – If the slope is without friction, and she is pulled up the slope at a constant speed, what will be the tension in the rope? – Considering the practical case where friction is present, suppose the tension in the rope were actually 100 N. What is the actual mechanical advantage of the slope? What would the efficiency be? Answers • A. 10/1= 1 • 50 N • 500/100=5 then 5/10= .5 which equals 50% Elastic potential Energy • PEelastic= 1/2kx2 – k= spring constant – x= extension or compression of the spring measured from equilibrium.